Graphics software programs are well known in the art. A graphics program consists of commands used to specify the operations needed to produce interactive three-dimensional images. It can be envisioned as a pipeline through which data pass, where the data are used to define the image to be produced and displayed. The user issues a command through the central processing unit of a computer system, and the command is implemented by the graphics program. At various points along the pipeline, various operations specified by the user's commands are carried out, and the data are modified accordingly. In the initial stages of the pipeline, the desired image is framed using geometric shapes such as lines and polygons (usually triangles), referred to in the art as “primitives.” The vertices of these primitives define a crude shell of the objects in the scene to be rendered. The derivation and manipulation of the multitudes of vertices in a given scene, entail performing many geometric calculations.
In the next stages, a scan conversion process is performed to specify which picture elements or “pixels” of the display screen, belong to which of the primitives. Many times, portions or “fragments” of a pixel fall into two or more different primitives. Hence, the more sophisticated computer systems process pixels on a per fragment basis. These fragments are assigned attributes such as color, perspective (i.e., depth), and texture. In order to provide even better quality images, effects such as lighting, fog, and shading are added. Furthermore, anti-aliasing and blending functions are used to give the picture a smoother and more realistic appearance. The processes pertaining to scan converting, assigning colors, depth buffering, texturing, lighting, and anti-aliasing are collectively known as rasterization. Today's computer systems often contain specially designed rasterization hardware to accelerate 3-D graphics.
In the final stage, the pixel attributes are stored in a frame buffer memory. Eventually, these pixel values are read from the frame buffer and used to draw the three-dimensional images on the computer screen. One prior art example of a computer architecture which has been successfully used to build 3-D computer imaging systems is the Open GL architecture invented by Silicon Graphics, Inc. of Mountain View, Calif.
Currently, many of the less expensive computer systems use its microprocessor to perform the geometric calculations. The microprocessor contains a unit which performs simple arithmetic functions, such as add and multiply. These arithmetic functions are typically performed in a floating point notation. Basically, in a floating point format, data is represented by the product of a fraction, or mantissa, and a number raised to an exponent; in base 10, for example, the number “n” can be presented by n=m.times.10.sup.e, where “m” is the mantissa and “e” is the exponent. Hence, the decimal point is allowed to “float.” Hence, the unit within the microprocessor for performing arithmetic functions is commonly referred to as the “floating point unit.” This same floating point unit can be used in executing normal microprocessor instructions as well as in performing geometric calculations in support of the rendering process. In order to increase the speed and increase graphics generation capability, some computer systems utilize a specialized geometry engine, which is dedicated to performing nothing but geometric calculations. These geometry engines have taken to handling its calculations on a floating point basis.
Likewise, special hardware have evolved to accelerate the rasterization process. However, the rasterization has been done in a fixed point format rather than a floating point format. In a fixed point format, the location of the decimal point within the data field for a fixed point format is specified and fixed; there is no exponent. The main reason why rasterization is performed on a fixed point format is because it is much easier to implement fixed point operations in hardware. For a given set of operations, a fixed point format requires less logic and circuits to implement in comparison to that of a floating point format. In short, the floating point format permits greater flexibility and accuracy when operating on the data in the pipeline, but requires greater computational resources. Furthermore, fixed point calculations can be executed much faster than an equivalent floating point calculation. As such, the extra computational expenses and time associated with having a floating point rasterization process has been prohibitive when weighed against the advantages conferred.
In an effort to gain the advantages conferred by operating on a floating point basis, some prior art systems have attempted to perform floating point through software emulation, but on a fixed point hardware platform. However, this approach is extremely slow, due to the fact that the software emulation relies upon the use of a general purpose CPU. Furthermore, the prior art software emulation approach lacked a floating point frame buffer and could not be scanned out. Hence, the final result must be converted back to a fixed point format before being drawn for display. Some examples of floating point software emulation on a fixed point hardware platform include Pixar's RenderMan software and software described in the following publications: Olano, Marc and Anselmo Lastra, “A Shading Language on Graphics Hardware: The PixelFlow Shading System,” Proceedings of SIGGRAPH 98, Computer Graphics, Annual Conference Series, ACM SIGGRAPH, 1998; and Anselmo Lastra, Steve Molnar, Marc Olano, and Yulan Wang, “Real-Time Programmable Shading,” Proceedings of the 1995 Symposium of Interactive 3D Graphics (Monterey, Calif., Apr. 9-12, 1995), ACM SIGGRAPH, New York, 1995.
But as advances in semiconductor and computer technology enable greater processing power and faster speeds; as prices drop; and as graphical applications grow in sophistication and precision, it has been discovered by the present inventors that it is now practical to implement some portions or even the entire rasterization process by hardware in a floating point format.
In addition, in the prior art, data is stored in the frame buffer in a fixed point format. This practice was considered acceptable because the accuracy provided by the fixed point format was considered satisfactory for storage purposes. Other considerations in the prior art were the cost of hardware (e.g., memory chips) and the amount of actual physical space available in a computer system, both of which limited the number of chips that could be used and thus, limited the memory available. Thus, in the prior art, it was not cost beneficial to expand the memory needed for the frame buffer because it was not necessary to increase the accuracy of the data stored therein.
Yet, as memory chips become less expensive, the capability of a computer system to store greater amounts of data increases while remaining cost beneficial. Thus, as memory capacity increases and becomes less expensive, software applications can grow in complexity; and as the complexity of the software increases, hardware and software designs are improved to increase the speed at which the software programs can be run. Hence, due to the improvements in processor speed and other improvements that make it practical to operate on large amounts of data, it is now possible and cost beneficial to utilize the valuable information that can be provided by the frame buffer.
Also, it is preferable to operate directly on the data stored in the frame buffer. Operating directly on the frame buffer data is preferable because it allows changes to be made to the frame buffer data without having to unnecessarily repeat some of the preceding steps in the graphics pipeline. The information stored in the frame buffer is a rich source of data that can be used in subsequent graphics calculations. However, in the prior art, some steps typically need to be repeated to restore the accuracy of the data and allow it to be operated on before it is read back into the frame buffer. In other words, data would need to be read from the frame buffer and input into the graphics program at or near the beginning of the program, so that the data could be recalculated in the floating point format to restore the required precision and range. Thus, a disadvantage to the prior art is that additional steps are necessary to allow direct operation on the frame buffer data, thus increasing the processing time. This in turn can limit other applications of the graphics program; for example, in an image processing application, an image operated on by the graphics program and stored in the frame buffer could be subsequently enhanced through direct operation on the frame buffer data. However, in the prior art, the accuracy necessary to portray the desired detail of the image is lost, or else the accuracy would have to be regenerated by repeated passes through the graphics pipeline.
Another drawback to the prior art is the limited ability to take advantage of hardware design improvements that could be otherwise employed, if direct operation on the frame buffer without the disadvantages identified above was possible. For example, a computer system could be designed with processors dedicated to operating on the frame buffer, resulting in additional improvements in the speed at which graphics calculations are performed.
Consequently, the use of fixed point formatting in the frame buffer is a drawback in the prior art because of the limitations imposed on the range and precision of the data stored in the frame buffer. The range of data in the prior art is limited to 0 to 1, and calculation results that are outside this range must be set equal to either 0 or 1, referred to in the art as “clamping.” Also, the prior art does not permit small enough values to be stored, resulting in a loss of precision because smaller values must be rounded off to the smallest value that can be stored. Thus, the accuracy of the data calculated in the graphics pipeline is lost when it is stored in the frame buffer. Moreover, in the prior art, the results that are calculated by operating directly on the data in the frame buffer are not as accurate as they can and need to be. Therefore, a drawback to the prior art is that the user cannot exercise sufficient control over the quality of the frame buffer data in subsequent operations.
Thus, there is a need for a graphical display system which predominately uses floating point throughout the entire geometry, rasterization, and frame buffering processes. The present invention provides one such display system. Furthermore, the display system of the present invention is designed to be compatible to a practical extent with existing computer systems and graphics subsystems.